Concept study GOME-2/AVHRR radiance inter-comparison Ruediger Lang, Rose Munro, Yakov Livschitz, Joerg Ackermann EUMETSAT Barry Latter NCEO/RAL

Outline Motivation Approach Preliminary results by NCEO/RAL (Barry Latter, et al.) Preliminary results by EUMETSAT GOME-2/Metop-A instrument and level 0 to 1B radiometric calibration team at EUMETSAT: Rose Munro (Mission scientist, PRD/MET) Ruediger Lang (Product expert, OPS/MOD) Yakov Livschitz (Software engineer, OPS/MOD) Antoine Lacan (Instrument engineer, LEO) Richard Dyer (Instrument engineer, OPS/CCD) AVHRR/Metop-A Joerg Ackermann (Product expert, OPS/MOD) NOEC/RAL: Member of GOME-SAG (B. Kerridge, R. Siddans, Barry Latter, et al.)

Motivation Radiometric accuracy: Day-one and instrument anomaly offset Long-term in-orbit changes (both nominal and non-nominal) Triggered by GOME-2 in-orbit throughput degradation Instrument alignment and geo-location pointing: Identify miss-alignments or pointing errors. Triggered by RAL study results. Investigation by EUMETSAT started in April.

GOME-2 / AVHRR radiance co-location GOME-2 channel 3 and 4 and AVHRR channel 1 data AVHRR ch1 response function GOME-2 Ch:4 (Band 6) 1024 measurements GOME-2 Ch:3 (Band 5) 1024 measurements

GOME-2 / AVHRR radiance co-location GOME-2 channel 3 and 4 and AVHRR channel 1 data AVHRR ch1 response function GOME-2 Ch:4 (Band 6) 1024 measurements GOME-2 Ch:3 (Band 5) 1024 measurements Detector pixel read-out sequence direction GOME-2 channel 3 Detector pixel read-out sequence direction GOME-2 channel 4 Read-out of all 1024 detector pixel takes 1024*45,78 ms = 46.875 ms introducing “spatial aliasing” (~2km across-track)

AVHRR/3 Validation using GOME-2 Co-located GOME-2 and AVHRR signals Courtesy of Barry Latter et al. (2009) AVHRR/3 geo-shifted GOME-2 ground pixel Wavelength (mm)

AVHRR/3 Validation using GOME-2 Scatter and offset before/after geo adjustment Nominal co-location GOME-2 Target Reflectance AVHRR/3 Target Reflectance GOME-2 Target Reflectance Adjusted co-location Across track: 20% FOV (~2 km) Along track: 6% FOV (~ 2.2 km) Courtesy of Barry Latter et al. (2009) 14th May 2008 Across Track shift is explained by GOME-2 read-out period (geo-location refers to start of the spectrum) Along Track shift is very likely a real geo-location difference (investigations ongoing!) GOME-2 convoluted target reflectancies are higher by about 10 % (relative value)

AVHRR/3 Validation using GOME-2 GOME-2 scanner position dependent geo-offset (in 2*%FOV) Courtesy of Barry Latter et al. (2009) AVHRR channel1 14th May 2008 Across-track offset between GOME-2 and AVHRR can be explained by “spatial aliasing” Along track offset however is not explained yet! potential miss-pointing by GOME-2 AVHRR Channel2 Only indicative because GOME ch4 covers only 20% of AVHRR ch2 plus significant water vapour contamination!!! East West East West

AVHRR/3 Validation using GOME-2 Long-term radiance inter-comparison Courtesy of Barry Latter et al. (2009)

GOME-2 Reflectivity degradation rate Sahara – 541nm – relative to mean of 2007 Reflectivity Sahara Normalised to 2007 Every 2nd scanner angle position is shown East West

GOME-2 Reflectivity degradation rate Sahara – 745nm – relative to mean of 2007 Reflectivity Sahara Normalised to 2007 Every 2nd scanner angle position is shown East West

Open questions and conclusions According to the RAL long-term results, there is no significant GOME-2 degradation in reflectivity for both AVHRR and GOME-2 in the AVHRRch1/GOMEch34 region visible yet What is the initial (at launch) offset for both instruments? According to the MODIS/AVHRR inter-comparison AVHRR (NOAA-16/17) experiences a dark bias of 9% (Cao, C., X. Xiong, A. Wu, and X. Wu, 2008, Assessing the consistency of AVHRR and MODIS L1B reflectance for generating fundamental climate data records, Journal of Geophysical Research, 1, 113, D09114, doi:10.1029/2007JD009363) Consistent with RAL results for GOME-2/AVHRR, but EUM results show opposite bias of same magnitude (bug in EUM analysis!?). For GOME-2 the initial offset in the solar mean reference has been estimated to be on the order of 1 to 3% in the AVHRRch1/GOMEch34 region (w.r.t. Wehrli 1985 SM0 reference spectrum). Reflectivity long-term degradation in the 640 nm region for GOME-2 not yet quantified (only at 541 and 745 nm).

The End

Reprocessed signals PPF 4.0 until February 2010 GOME-2 Long-term throughput changes Solar Mean Reference (SMR) spectrum Reprocessed signals PPF 4.0 until February 2010 relative to February 2007

Courtesy of Barry Latter et al. (2009) AVHRR/3 Validation using GOME-2 GOME-2 scanner position dependent geo-offset (in 2*%FOV) Courtesy of Barry Latter et al. (2009) AVHRR channel 1 14th May 2008 Across-track [2*%FOVGOME ] Along-track [2*%FOVGOME ] East West GOME-2 scanner position